Modify node value in all materials with variable name?
You want to iterate over all materials and check each material's name. IF it starts with 'mb:' then modify it.
import bpy 
for material in bpy.data.materials:
    if material.name.startswith(&quot;mb:&quot;):
        material.node_tree.nodes[&quot;Value&quot;].outputs[0].default_value = 10